1. Field of the Invention
The present invention relates to a method for channel estimation in a communication system, particularly to a method for OFDM and OFDMA channel estimation via phase-rotated polynomial interpolation and extrapolation.
2. Description of the Prior Art
There exist various electronic products today for wireless communication, including mobile phones, wireless metropolitan area network (WMAN), wireless local area network (WLAN), global positioning system (GPS), Bluetooth, and so on. Besides convenience of use, size, and external appearance, their performance is unsurprisingly a focus point that must be attended to in associated research and development. It also goes without saying that, in this, the technology relating to baseband signal transmitting and receiving components, which play a critical role in the overall transceiver operation, is of great importance.
Channel estimation algorithms play an important role in transceiver design for wireless communication systems based on orthogonal frequency-division multiplexing (OFDM), orthogonal frequency-division multiple access (OFDMA), etc. Various forms of channel estimation algorithms can be designed for various pilot carrier (also called pilot subcarrier) configurations and various channel characteristics, resulting in a significant number of channel estimation algorithms employing various methodologies and with various realization complexities.
United States Patent Application No. 2007/0183521 titled “Method of channel estimation” discloses a technique that divides a frequency response into an amplitude response and a phase response for channel estimation. It estimates the amplitude response and the phase response separately and combines them into a channel response. Various interpolation and extrapolation (henceforth abbreviated inter/extra-polation) methods, e.g. linear inter/extra-polation or various weighted inter/extra-polation methods, can be used in the estimation of amplitude and phase responses. Because of the need to estimate amplitude and phase responses, this method requires the non-linear coordinate conversion from polar coordinates to rectangular coordinates for each subcarrier where the channel response is to be estimated.
United States Patent Application No. 2006/0239178 A1 titled “Reduced complexity channel estimation in OFDM systems” considers the processing of received signal. Observing that transmission delay causes phase rotation in the received signal, the method counter-rotates the received signal at each subcarrier to a channel delay appropriate for channel estimation prior to performing the channel estimation for the subcarriers. The algorithm seeks to lower the effect of signal phase rotation before channel estimation. It is based on heuristic reasoning without a mathematically clearly formulated quality measure and it needs to apply a different amount of phase rotation to each subcarrier.
M.-H. Hsieh and C.-H. Wei's “Channel estimation for OFDM systems based on come-type pilot arrangement in frequency selective fading channels” (IEEE Trans. Consumer Elec., vol. 44, no. 1, pp 217-225, February 1998) proposed a linear interpolation with phase compensation for combined multicarrier symbol timing and channel response estimation to mitigate the linear interpolation error caused by channel transmission delay. However, the amount of phase rotation is estimated in an ad hoc and heuristic manner, lacking a proper theoretical foundation and neither being an optimal solution. In contrast, the present invention derives the theoretically optimal values and proposes several methods to estimate the optimal values.
In addition, for linear inter/extra-polation and, more generally, for polynomial inter/extra-polation, the performance depends on the accuracy of symbol timing estimation and the amount of channel delay spread. Inaccurate symbol timing estimation results in relatively long channel delay, which, similar to excessive channel delay spread, would cause severe model error in linear inter/extra-polation and general polynomial inter/extra-polation. This phenomenon is an important design issue in system implementation. Take OFDMA uplink transmission, for example. Because multiple users transmit their symbols according to a common timing plan whereas different users experience different propagation delays, a ranging process is often employed to adjust the uplink transmission delays so as to attain accurate symbol timing synchronization among all users. If the channel estimation algorithm could take into consideration the effect of different propagation delays of different users and handle it properly, then it could reduce the required accuracy in symbol time synchronization of the ranging process.